This blog is a lab notebook for my work with the Reprap open source 3D printing undertaking.

Tuesday, March 02, 2010

Printing light structures

A month ago, I designed a light frame structure which I could post tension and use to replace much of the steel in a next generation Reprap printer.

Processing it into gcode with Skeinforge proved to be impossible. That led me into a meandering sidetrack which had me pushing my old Slice and Dice code into working order after several years of neglect after Netfabb ran late with the release date for their STL to gcode processing module. Last weekend, I got Slice and Dice working acceptably.

After work yesterday afternoon, I decided to see if I could make my Slice and Dice alpha code successfully generate gcode for a print of that same light structure. There were a few hiccoughs. Fortunately, Slice and Dice is purposely designed to let me deal with difficult prints. I was able to identify problems by doing partial prints and tweak my settings to get past them.

When I got up this morning I finished processing the light structure STL and set my Rapman printer to work on it. Being a stand-alone 3D printer, it happily worked while I got on with my day job. Roughly 90-100 minutes later, I had a printed module.

I was quite chuffed with my design. Clearing the female connectors on the bottom with a 5/32" bit took a matter of a few seconds and let me mate one of my test prints of the top of the module with the whole printed module. I immediately started printing up a second module. It finished up a few moments ago.

I think it's pretty obvious where this is going. I did a short clip of the print underway so you can get an idea of how fast it proceeds.

I am printing a third module now and plan on printing five over the course of the day. This evening after work, I plan on designing end caps for the assembly so that I can post tension it and then get an idea of how much deflection it exhibits under load for a 500 mm beam.

The lessons I've learned from all this is that if you design light and print vertically, viz, keep the cross-section on the xy print table small, you are going to get what are basically no-warp parts without going through the drama of heated print beds or turkey bags. it's worth thinking about.

16 comments:

If the four posts are hollow (I think they are), would it be possible to use four lengths of 3mm ABS (or PLA, or whatever you have in hand) to thread and post-tension each of the four posts in the final structure?

Less vitamins, less parts to buy, cheaper... perhaps one M3 nut in each end of each 3mm rod could be used to adjust the tension.

Oh, I'll introduce myself: I am Alvaro Deibe Diaz, from Spain. We are begining to (try to) print things with our first Darwin. We have, also, a beatiful set of plastic parts to make a new Mendel.

I've been using very thin threaded rod ~3-4 mm centrally located to do the post tensioning. The columns are hollow. I just checked and the void is only about 2 mm. The module could be sized up a bit to accomodate a 3 mm ABS filament I suspect. How you plan to put it tension escapes me, though.

Very cool Forestt. How were you thinking you'd permanently attach two of these together? Some form of adhesive? Or some kind of mechanical catch that would snap together and then require much greater force to remove?

I thought that, perhaps, threading one M3 nut directly at each end of each plastic rod would be enough. The inside diameter of an M3 nut is around 2.6mm (and our PLA rod is 2.8-2.9mm width).

The idea would be to have one piece of 3mm plastic rod trough the holes of each of the four corners of the structure. Each rod could be tensioned with two nuts, one screwed directly at each end of the rod.

I see what you are saying, now. I wouldn't think that that would work. There is only one way to be sure, though, and that is to try it and see for yourself. Fortunately, having a Darwin you can do that. :-)

First I threaded one end of a 300 mm long PLA filament, with an M3 die. Then added an M4 washer, and hanged the filament from the nut and washer, in a vice, and added weight to the other end of the PLA. The thread failed at 18kg (~40 pounds).

With the thread made by the die, the nut screwed in the PLA very softly, so I assumed the die cut too deeply into the plastic. So I used the M3 nut to directly make a new thread in the PLA. This time the nut screwed much more tightly.

Again, adding weight to the filament, hanged from the washer and nut, the PLA failed at 18kg. But the nut keep firmly screwed there. The fail was on the other end of the PLA! (the weight was hanged from an "o" made with the PLA filament; that placed too much stress in it). It seems like the thread can keep up to the tensile strength of the filament.

I think it is easily possible to hang 20-25 kg from the PLA filament, keeping it straight, and pressumible more from ABS (I can't test this, though).

If my numbers are right, your 40 cm long and 3 cm width beam, post tensioned with four PLA filament, one in each column, could hold a point load of around 7 kg (~15 pounds), or more from ABS.

I have some photos to illustrate the "hanging", but I don't see how to include them...

Howard23: I read over your blog entry and there's nothing at all to be ashamed of in it. You described exactly what you did and took excellent photographs to reinforce your descriptive text. I wish more people were as careful in their blogging as you obviously are.